This invention relates to a helical antenna which can be used in a digital radio receiver and the like.
In recent years, a digital radio receiver for receiving a radio wave from an artificial satellite (which may be called a xe2x80x9csatellite wavexe2x80x9d hereinafter) or a radio wave from a ground station (which may be called a xe2x80x9cground wavexe2x80x9d hereinafter) to listen to digital radio broadcasting has been developed and is about to be put into practical use. The digital radio receiver is mounted on a mobile station such as a vehicle and is adapted to receive a radio wave having a frequency of about 2.3 GHz to listen to the digital radio broadcasting. In other words, the digital radio receiver is a radio receiver capable of listening to mobile broadcasting. It is noted here that the ground wave is a radio wave obtained by slightly shifting the frequency of the satellite wave after it is received by the ground station.
In order to receive the radio wave having the frequency of about 2.3 GHz, it is necessary to mount an antenna at a position outside a vehicle. Such antenna may have various structures, but a stick type is generally used rather than a planar type (or a flat type).
As is well known, an electromagnetic wave emitted into a free space is a transversal wave having an electric field and a magnetic field vibrating in a plane perpendicular to a propagating direction of the wave. The electric field and the magnetic field are variable in intensity within the above-mentioned plane. Such an electromagnetic wave in which the direction of the electric field is not random but constant or varied in some regular way is referred to as a polarized wave. The satellite wave is a circular polarized wave exhibiting circular polarization while the ground wave is a linear polarized wave exhibiting linear polarization.
As one of the stick-type antennas, a helical antenna is known. The helical antenna comprises a hollow or solid cylindrical body and a conductor wire wound around the cylindrical body in a helical shape (or a spiral shape) and can efficiently receive the above-mentioned circular polarized wave. Therefore, the helical antenna is exclusively (or mainly) used to receive the satellite wave.
It is unavoidable for an automobile to vibrate when moving. Therefore, the helical antenna to be mounted on the automobile must be provided with a sufficient anti-vibration measure. It is needless to say that the helical antenna is required to be excellent in durability.
It is therefore an object of this invention to provide a helical antenna which is excellent in vibration resistance and which is improved in durability.
It is another object of this invention to provide a helical antenna which is improved in workability in an assembling operation.
Other objects of the present invention will become clear as the description proceeds.
According to the present invention, there is provided a helical antenna which comprises a cylindrical body formed by an insulating film member which is rolled into a cylindrical shape, an antenna pattern extending in a helical shape along said cylindrical body, a circuit board opposite to one of the axial ends of the cylindrical body, a circuit pattern formed on the circuit board; a connecting portion connecting the antenna pattern with the circuit pattern by soldering, and a fixed portion fixing the circuit board to a specific part of the cylindrical body, wherein the specific part is not provided with the antenna pattern.